For full functionality, you must use “pretty” permalinks with The Events Calendar and Event Tickets. Without getting too technical, some of the functionality of the plugins depends on these more-readable URLs in order to work.
Go to Settings → Permalinks. Select the style of permalink that you want. We recommend Post Name.
💡 Pro tip: If you get a “Page Not Found” (or 404) error when visiting the calendar, try clicking the “Save Changes” button on the Permalink Settings page without making any changes. This flushes the site’s cache and often resolves those issues.